1. Field of the Invention
The present invention relates to a process for preparing aromatic bisphosphates. More particularly, the present invention relates to a process for preparing highly pure aromatic bisphosphates having an extremely low content of oligomeric phosphates and triphosphate components, capable of being solidified easily, and being useful as a fire retardant or a plasticizer for a synthetic resin.
2. Description of the Related Arts
Aromatic bisphosphates have characteristics such as low volatility, high temperature stability and no poisonous halogens, so that they are used as a fire retardant or plasticizer for imparting fire retardancy, temperature stability and good moldability to thermoplastic resin or thermosetting resin. Also, aromatic bisphosphates have a resistance to the temperature (about 300.degree. C.) for molding highly functional plastics that have been recently developed.
There are known two methods for preparing aromatic bisphosphates. One method (1) involves reacting an aromatic monohydroxy compound with a phosphorus oxyhalide and then reacting the resultant with an aromatic dihydroxy compound. The other method (2) involves reacting an aromatic dihydroxy compound with a phosphorus oxyhalide and then reacting the resultant with an aromatic monohydroxy compound.
In the method (1), when an aromatic monohydroxy compound having no steric hindrance group at its ortho position is reacted with phosphorus oxychloride, monoarylphosphorodihalidate and triarylphosphate are produced as by-products in addition to the object diarylphosphorohalidate, and also unreacted phosphorus oxyhalide is left as a residue. If the reaction mixture obtained as above without purification is reacted with an aromatic dihydroxy compound, it is not possible to obtain an aromatic bisphosphate of high purity. On the other hand, if the reaction mixture is subjected to a purification process such as distillation, the yield will be extremely lowered, which is economically disadvantageous.
Alternatively, if an aromatic monohydroxy compound having a steric hindrance group at its ortho position is reacted with phosphorus oxyhalide in the method (1), it is not possible to prepare an aromatic bisphosphate of high purity although the aforementioned by-products are produced at a lower ratio.
In the method (2), if a theoretical amount of phosphorus oxyhalide is reacted with an aromatic dihydroxy compound, it is inevitable that an oligomeric phosphate is produced as a by-product. Therefore, the compound obtained by reacting the reaction mixture containing the by-product with an aromatic monohydroxy compound contains a large amount of the oligomeric phosphate, so that the obtained compound will have a resin-like shape. An excess amount of phosphorus oxyhalide used in the reaction reduces only the ratio of the oligomeric phosphate produced as a by-product and does not produce an aromatic bisphosphate of high purity. In addition, the unreacted phosphorus oxyhalide in the reaction mixture must be removed before the reaction with an aromatic monohydroxy compound. Accordingly, the method (2) is not an economical and efficient manufacturing process.
The oligomeric phosphate produced as a by-product is not preferable because it lowers the purity of the object aromatic bisphosphate and, therefore, prolongs the time required for solidification of the aromatic bisphosphate in a melted state after the reaction or makes it difficult to solidify the aromatic bisphosphate. Also, the triaryl phosphate produced as a by-product is not preferable because it is poor in heat resistivity and causes plate-out at the time of molding the resin, contaminating the surface of the obtained molded article or degrading the appearance.
In order to obtain an aromatic bisphosphate of high purity, it is necessary to lower the content of these by-products. However, there were no processes that could reduce the content of these by-products simultaneously.
The above-described method (2), i.e. the process of preparing an aromatic bisphosphate having a steric hindrance group at its ortho position, is disclosed in the U.S. Pat. No. 5,420,327. In an Example of the above U.S. patent, an aromatic bisphosphate having a purity of 85.0 to 93.9% as measured by High Pressure Liquid Chromatography (HPLC) is obtained by reacting a little excess amount of 2,6-xylenol with phosphorus oxychloride to prepare bis(2,6-xylenyl) phosphorochloridate and reacting the obtained reaction mixture with hydroquinone without subjecting the reaction mixture to separation by distillation. However, the above U.S. patent does not mention the molar ratio nor the composition ratio of the by-products in the reaction of the aromatic monohydroxy compound with phosphorus oxychloride.
Japanese Unexamined Patent Publication (Kokai) No. HEI 8(1996)-143584 discloses a process of preparing a diarylphosphorohalidate compound of high purity by reacting 2,6-xylenol with phosphorus oxyhalide in the presence of a nitrogen-containing heterocyclic compound catalyst. However, the above Japanese article does not mention a purification process by distillation.